1. Field of the Invention
The present invention relates to an image pickup device adapted for use in a video camera or the like.
2. Related Background Art
Recent remarkable progress in imaging equipment such as video cameras has realized automation of various functions and improvement in operability, such as incorporation of zoom lens, automatic focusing and automatic exposure control. For example, the automatic exposure control is an extremely important factor governing the quality of the obtained image, and has to always function stably and satisfactorily in any phototaking condition.
FIG. 1 is a block diagram showing a basic configuration of the exposure control system of an ordinary video camera, wherein provided are a phototaking lens optical system 101; an iris (diaphragm) 102 for regulating the amount of incident light; a photosensor device 103 such as a CCD, for effecting photoelectric conversion on an image, which is focused by the phototaking optical system on a phototaking face of said device and is regulated in the light amount by the iris, into an image signal; a camera signal processing circuit 104 for applying a predetermined signal processing to the image signal released from said photosensor device thereby obtaining a standardized image signal; an image signal output terminal 105; a motor 106 for driving the iris 102 for varying the aperture thereof; an iris driving circuit 107 for controlling the motor 106; a CCD drive circuit 108 for controlling the timing of accumulation, signal readout and resetting of the photosensor device 103 and varying the accumulation time (exposure time) of said device thereby obtaining a desired shutter speed; an automatic exposure control (AE) circuit 109 for evaluating the exposure state, based on the luminance signal from the camera signal processing circuit, and obtaining an optimum exposure by controlling the iris drive circuit 107 and the CCD drive circuit 108; and a switch panel 110 for entering key operations.
The exposure control by the AE circuit 109 is conducted in the following manner. There is formed an iris controlling closed loop, for integrating the luminance signal from the signal processing circuit 104, controlling the iris driving circuit 107 so as to maintain the level of said signal within a predetermined range and controlling the drive current to the iris motor for varying the aperture of the iris, and there is provided a control system for controlling the CCD drive circuit 108 to switch the driving pulses thereof in response to the key operation on the switch panel 110, thereby varying the accumulation time of the image pickup device 103 to control the exposure time and to obtain an appropriate exposure. Said accumulation time control is called electronic shutter, and can select several stages from 1/100 sec. to 1/10000 sec., in addition to the ordinary exposure time of 1/60 sec. for NTSC standard.
When a high-speed electronic shutter is used in such system, there is assumed so-called shutter priority mode in which the iris is controlled according to an arbitrarily selected shutter speed. FIG. 2 shows said shutter priority mode, in which the shutter speed in the abscissa is at first fixed, and the aperture value in the ordinate is varied accordingly.
However, the above-explained iris control in the shutter priority mode according to the luminance level of the image signal as in the foregoing video camera is unable to provide appropriate exposure control under various phototaking conditions.
In a camera for still image taking, such as a conventional still camera utilizing a silver halide film, the exposure control needs to be appropriate only at the moment of phototaking, but, in case of recording the moving image for a long time as in the video camera, the exposure control has to be conducted in constantly stable and optimum manner, following the continuously varying conditions in the course of phototaking operation, and an exposure control device for video camera, meeting these requirements, has been longed for.